(1) Field of the Invention
The present invention is an acoustically transparent carbon nanotube thermophone.
(2) Description of the Prior Art
Traditional acoustic transduction typically begins with the generation of electrical excitation pulsed through an amplifier into an electro-acoustic material (such as a piezoelectric ceramic, piezocomposite, or a magnetostrictive ferromagnetic compound). This electrical excitation creates a mechanical vibration that is then converted into an acoustic wave to produce sound. The lower the preferred transmitting frequency (and hence a longer acoustic detection range) desired; the larger the size of the conventional transducer that is required. Often, for acoustic projectors producing sound at frequencies below a few kHz; the electroacoustic device needs to be comparatively very large in order to produce long sound waves. The large size of the electroacoustic device can be a serious limitation for incorporating low-frequency, long-range detection sonars on autonomous underwater vehicles (AUVs).
When using comparatively smaller or thin conductors, alternating current is passed through the conductor. Following variations in current strength; periodic heating takes place in the conductor. This periodic heating produces temperature waves which are propagated into a surrounding liquid medium. The amplitude of the temperature waves fall off rapidly as a distance from the conductor increases. Because of the rapid variations of these temperature waves; the net effect is to produce a periodic rise in temperature in a portion of the gas/liquid medium near the conductor. Thermal expansion and contraction of this portion (or layer) of the medium determines the amplitude of the resulting sound waves.
Recently, there has been development of underwater acoustic carbon nanotube (CNT) yarn sheets capable of producing high acoustic output at low frequencies with broad bandwidth. An underwater acoustic transmitter is feasible in which the transmitter uses thermal means to heat CNT substrates and in which a low frequency acoustic projector is formed. The acoustic carbon nanotubes can act as transducers while having a comparatively small volumetric size. The principle transduction for this approach is through thermal acoustics as opposed to conventional underwater transducers that utilize electromechanical vibrations. This type of thermal acoustic producing device is known as a thermophone.
However, a problem with using the thermal acoustics of carbon nanotubes is that the nanotubes (unless encapsulated or housed) are quite fragile and are susceptible to disintegration especially if the nanotubes are touched or moved too quickly. A bare nanotube configuration also has a serious risk of damage when being transported and handled.
Another problem is using the CNT yarn sheets in a high power conventional wave operation in water. This type of use overheats the CNT sheets such that the housing encapsulation temperature may rise to an unacceptable level that can induce thermal damage in the housing. As such, when making a CNT thermophone, encapsulating materials need capabilities for high temperature and heat dissipation.